The subject matter disclosed herein relates generally to propulsion systems and to, in particular, a rotary wing aircraft having an electrified propulsion system.
Rotary wing aircraft utilize propulsion systems to power aircraft flight. These propulsion systems convert stored energy into mechanical work to drive one or more rotor systems for flight. Energy (typically stored in chemical form as fuel) is supplied to an energy conversion device (typically a plurality of internal combustion engines such as a turbine engine, spark ignition engine, or compression ignition engine), which converts the energy into mechanical work. A drive system transmits mechanical work through a plurality of transmission mechanisms (e.g., main rotor gearbox(es), a tail rotor gearbox, auxiliary propulsor gearbox(es), drive shafts, drive couplings, etc.) to drive the rotary wing aircraft's thrust generating rotors. As these mechanical transmission devices transmit mechanical power from the rotorcraft engines along a chain of components, they experience a large number of fatigue cycles. Each component designed for fatigue life is typically overdesigned to meet the component life requirement, which adds weight to the component. Each component in the chain that physically engages another such as, e.g., gears and bearings, generates heat from friction and requires lubrication to minimize friction losses and a cooling system to reject heat. Lubrication and cooling systems add weight to the conventional rotorcraft. Additionally, these mechanical transmission devices must also be supported by strengthened airframe structure which further increases vehicle weight. To change the direction of the mechanical transmission, angled bevel gears requiring thrust force reaction are needed. This thrust force reaction along the gear requires additional weight from bearings, shafts and housing structure.
Additionally, electricity for supplying the aircraft's electrical load may be supplied through a plurality of generators which are mounted to the main rotor gearbox(es) or engine(s). For a typical aircraft, the energy demand of the engine differs depending on the flight segment such as steady/level flight, take off, vertical landing, hover, or during emergency conditions, etc. As a result, the engine may not operate near its peak efficiency during off-design point power conditions. For missions where the rotorcraft demand is mostly for the off-design point power segment of flight, the rotorcraft may generally exhibit lower efficiency. A propulsion system for a rotary wing aircraft that provides optimal operating efficiency of the engine would increase the fuel efficiency of the vehicle and provide greater value to the operator of the aircraft. Furthermore, a propulsion system that reduces mechanical complexity would also decrease total vehicle weight, improve its reliability and reduce its maintenance burden to provide greater value to the operator of the aircraft.